Study on power generation and Congo red decolorization of 3D conductive PPy-CNT hydrogel in bioelectrochemical system

In this paper, a polypyrrole-carbon nanotube hydrogel (PPy-CNT) with 3D macroporous structure was prepared by secondary growth method. This self-supporting material with good conductivity and biocompatibility can be directly used as anode in a microbial fuel cell (MFC). The prepared material had a uniform structure with rich 3D porosity and showed good water retention performance. The effect of the mass ratio of PPy and CNT in the hydrogel were also investigated to evaluate the electrical performance of MFC. The MFC with 10:1 PPy-CNT hydrogel anode could reached the maximum power density of 3660.25 mW/m3 and the minimal electrochemical reaction impedance of anode was 5.06 Ω. The effects of Congo red concentration, external resistance and suspended activated sludge on decolorazation and electricity generation were also investigated in the MFC with the best performance hydrogel. When the Congo red concentration was 50 mg/L and the external resistance was 200 Ω, the dye decolorization rate and chemical oxygen demand (COD) removal rate could reach 94.35% and 42.31% at 48h while the output voltage of MFC was 480 mV. When activated sludge was present, the decolorization rate and COD removal rate could be increased to 99.55% and 48.08% at 48 h. The above results showed that the porous hydrogel anode had broad application prospects in synchronous wastewater treatment and electricity production of MFC.     

Long-term effect of carbon nanotubes on electrochemical properties and microbial community of electrochemically active biofilms in microbial fuel cells

Carbon nanotubes (CNTs) have been widely exploited to improve anodic performance, but information is needed on their long-term stability for improvement. Herein, we prepared a novel CNTs-modified graphite felt (CNTs-GF) by a simple and scalable process and evaluated its long-term performance using anaerobic sludge as inoculum. the MFC with CNTs-GF yielded a sustained enhancement of power output, increasing from 1.93 ± 0.09 W m^?2 after 1 month to 2.10 ± 0.05 W m^?2 after 3 months and reaching 2.00 ± 0.10 W m^?2 after 13 month, indicating the enhancement in electricity generation by the CNTs was not declined over one year. However, the bare GF showed a declining tendency of performance during 13 months. The long-term enhancement can be explained by the facts that the CNTs-GF was beneficial to electrochemically active biofilms (EABs) growth and interacted better with EABs and increased the extracellular electron transfer. Community analysis showed an increase in Geobacter in response to CNTs modification. These results demonstrated that CNTs modification could sustain a superior long-term enhancement in MFC performance.     

Iron chelates as low-cost and effective electrocatalyst for oxygen reduction reaction in microbial fuel cells

Iron-chelated electrocatalysts for the oxygen reduction reaction (ORR) in a microbial fuel cell (MFC) were prepared from sodium ferric ethylenediamine-N,N′-bis(2-hydroxyphenylacetic acid) (FeE), sodium ferric diethylene triamine pentaacetic acid (FeD) supported on carbon Vulcan XC-72R carbon black and multi-walled carbon nanotubes (CNTs). Catalyst morphology was investigated by TEM; and the total surfaces areas as well as the pore volumes of catalysts were examined by nitrogen physisorption characterization. The catalytic activity of the iron based catalysts towards ORR was studied by cyclic voltammetry, showing the higher electrochemical activity of FeE in comparison with FeD and the superior performance of catalysts supported on CNT rather than on Vulcan XC-72R carbon black. FeE/CNT was used as cathodic catalyst in a microbial fuel cell (MFC) using domestic wastewater as fuel. The maximum current density and power density recorded are 110 (mA m⁻²) and 127 ± 0.9 (mW m⁻²), respectively. These values are comparable with those obtained using platinum on carbon Vulcan (0.13 mA m⁻² and 226 ± 0.2 mW m⁻²), demonstrating that these catalysts can be used as substitutes for commercial Pt/C.     

Ni/NiO@rGO as an efficient bifunctional electrocatalyst for enhanced overall water splitting reactions

Herein, we fabricated bifunctional, noble metal-free, highly efficient nickel/nickel oxide on reduced graphene oxide (Ni/NiO@rGO) by chemical synthesis approach for electrochemical water splitting reaction. Its structural and morphological characterization using thermogravimetric analysis (TGA), transmission electron microscopy (TEM), field emission scanning electron microscope (FESEM), energy dispersive analysis of X-ray (EDAX) and X-ray diffraction (XRD) represents, Ni/NiO@rGO is having Ni/NiO NPs ∼10 nm (±2 nm) on graphene oxide with face-centered cubic (FCC) crystal structure. Moreover, the presence of Ni/NiO (2.26%), O (6.56%), N (0.74%) and C (90.44%) from EDAX analysis further confirms the formation of Ni/NiO@rGO and it also supported by FTIR studies. This nanocatalyst is examined further for electrocatalytic water splitting reactions (HER and OER). It demonstrated low overpotential 582 mV to achieve current density at 10 mA cm⁻² and smaller Tafel slope of 63 mV dec⁻¹ obtained in 0.5 M H₂SO₄ towards HER. Also, at the other end at onset potential of 1.6 V vs. RHE towards OER. It demonstrated low overpotential 480 mV to achieve current density at 10 mA cm⁻² and smaller Tafel slope of 41 mV dec⁻¹ in 0.5 M KOH towards OER observed. Hydrogen fuel is eco-friendly to the environment and noteworthy performance of earth-saving reactions.     

Power generation and organics removal from wastewater using activated carbon nanofiber (ACNF) microbial fuel cells (MFCs)

Carbon-based materials are the most commonly used electrode material for anodes in microbial fuel cell (MFC), but are often limited by their surface areas available for biofilm growth and subsequent electron transfer process. This study investigated the use of activated carbon nanofibers (ACNF) as the anode material to enhance bacterial biofilm growth, and improve MFC performance. Qualitative and quantitative biofilm adhesion analysis indicated that ACNF exhibited better performance over the other commonly used carbon anodes (granular activated carbon (GAC), carbon cloth (CC)). Batch-scale MFC tests showed that MFCs with ACNF and GAC as anodes achieved power densities of 3.50 ± 0.46 W/m³ and 3.09 ± 0.33 W/m³ respectively, while MFCs with CC had a lower power density of 1.10 ± 0.21 W/m³ In addition, the MFCs with ACNF achieved higher contaminant removal efficiency (85 ± 4%) than those of GAC (75 ± 5%) and CC (70 ± 2%). This study demonstrated the distinct advantages of ACNF in terms of biofilm growth and electron transport. ACNF has a potential for higher power generation of MFCs to treat wastewaters.     

Optimization of high-solid waste activated sludge concentration for hydrogen production in microbial electrolysis cells and microbial community diversity analysis

To enhance hydrogen recovery from high-solid waste activated sludge (WAS), microbial electrolysis cells (MECs) were used as an efficient device. The effects of WAS concentrations were firstly investigated. Optimal concentration for hydrogen production was 7.6 g VSS/L. Maximum hydrogen yields reached to 4.66 ± 1.90 mg-H₂/g VSS and 11.42 ± 2.43 mg-H₂/g VSS for MECs fed with raw WAS (R-WAS) and alkaline-pretreated WAS (A-WAS) respectively, which was much higher than that obtained traditional anaerobic digestion. Moreover, no propionic acid accumulation was achieved at the optimal concentration. Effective sludge reduction was also achieved in MECs feeding with A-WAS. 52.9 ± 1.3% TCOD were removed in A-WAS MECs, meanwhile, protein degradation were 50.4 ± 0.8%. The 454 pyrosequencing analysis of 16S rRNA gene revealed the syntrophic interactions were existed between exoelectrogen Geobacter and fermentative bacteria Petrimonas, which apparently drove the efficient performance of MECs fed with WAS.     

Enhanced methane fermentation of municipal sewage sludge by microbial electrochemical systems integrated with anaerobic digestion

Sewage sludge from a municipal wastewater treatment plant was fed into a microbial electrochemical system, combined with an anaerobic digester (MES-AD), for enhanced methane production and sludge stabilization. The effect of thermally pretreating the sewage sludge on MES-AD performance was investigated. These results were compared to those obtained from control operations, in which the sludge was not pretreated or MES integration was absent. The soluble chemical oxygen demand (SCOD) in the raw sewage sludge after pretreatment was 31% higher than the SCOD in untreated sludge (5804.85 mg/L vs. 4441.46 mg/mL). The methane yield and proportion of methane in biogas generated by the MES-AD were higher than those of the control systems, regardless of the pretreatment process. The maximum methane yield (0.28 L CH₄/g COD) and methane production (1139 mL) were obtained with the MES inoculated with pretreated sewage sludge. Methane yield and production with this system using pretreated sewage were 47% and 56% higher, respectively, than those of the control (0.19 L CH₄/g COD, 730 mL). Additionally, the maximum SCOD removal (89%) and current generation were obtained with the MES inoculated with a pretreated substrate. These results suggested that sewage sludge could be efficiently stabilized with enhanced methane production by synergistic combination of MES-AD system with pretreatment process.     

Integration of submersible microbial fuel cell in anaerobic digestion for enhanced production of methane and current at varying glucose levels

A submersible microbial fuel cell (SMFC) was coupled with anaerobic digestion (AD) system to establish synergy for enhancing the electricity and methane production at different glucose concentration of 2, 4 and 10 g/l. High amount of stable current generation of 0.35 mA was obtained at 4 g/l, which was about 1.5 times higher than the SMFC-AD operated at 10 g/l glucose. Methane production and yield were enhanced by 69% and 28%, respectively in SMFC-AD in comparison with AD operation at 2 g/l. Maximum methane yield of 0.32 l-CH₄/g COD was observed in SMFC-AD operation at 2 g/l, followed by 4 g/l (0.28 l-CH₄/g COD) and 10 g/l (0.18 l-CH₄/g COD). Furthermore, the SMFC-AD process increased COD removal and maintained proper pH of around 6.8–7.3 for efficient methane production. This study suggests that the SMFC-AD can achieve enhanced methane production compared to stand-alone AD with additional electricity generation.     

Bioelectricity recovery and pollution reduction of distillery wastewater in air-cathode SCMFC

Generation of electricity and treatment of the distillery wastewater (DW) were achieved in air-cathode single-chamber MFCs (SCMFCs). The maximum current density of 2.36 mA·m⁻² and power density of 39.21 mW·m⁻³ were obtained at DW concentration of 2000 mg COD.L⁻¹. The trends of current and power density were decreased with increasing concentration from 2000 to 4000 mg COD.L⁻¹. The maximum soluble COD removal was 62.2% (CE = 8.77%, 1500 mg COD.L⁻¹). The electrochemical activity of the microbes in the SCMFCs showed significantly high oxidized substrate in the semi-batch distillery wastewater operation. The microbial communities on the anode biofilms fed with a high concentration of distillery wastewater (4000 mg COD.L⁻¹) operation were analyzed based on 454 pyrosequencing of the 16S rRNA gene revealed different microbial communities. The dominant phylum were Proteobacteria (28.5%), Bacteroidetes (23.4%) and Chlorobi (22.8%), while Firmicutes (9.3%) were detected as a lesser proportion of the total community. The microbial community structures suggested that Proteobacteria might be important for the production of power density in distillery wastewater fed air-cathode SCMFC. These results demonstrate that the SCMFCs can simultaneously generate electricity and achieve wastewater treatment from a renewable source.     

In-situ hydrogen peroxide synthesis with environmental applications in bioelectrochemical systems: A state-of-the-art review

Hydrogen peroxide (H₂O₂) is a versatile, eco-friendly, strong oxidizing chemical with numerous industrial applications. It is found that bioelectrochemical system (BES) is a promising technology for H₂O₂ biosynthesis including microbial fuel cell (MFC) and microbial electrolysis cell (MEC), generally. Since first discovery of H₂O₂ production in BES in 2009, a growing community of researchers payed attention to on-site H₂O₂ production and environmental applications based on BESs. In this review, we discussed the state-of-the-art development, performance and environmental applications of H₂O₂-BES in detail. The H₂O₂-BES has been getting more and more energy-saving even turning “waste” into wealth completely without other energy input. Moreover, coupling the H₂O₂-BESs with Fenton and ultraviolet/visible light is extensively employed for environmental applications, ranging from dye decolorization, metal deposition, emerging contaminants, real wastewater and primary sludge treatment in lab-scale. However, the pilot- or industrial-scale applications of BESs are challenging enough in environmental remediation up to now.     

Elementary reaction modeling and experimental characterization of solid oxide fuel-assisted steam electrolysis cells

A one-dimensional elementary reaction kinetic model for solid oxide fuel-assisted steam electrolysis cell (SOFEC) is developed coupling heterogeneous elementary reactions, electrochemical reaction kinetics, electrode microstructure and transport processes of charge and mass. This model is calibrated and validated by experimental data from a button cell with anode gases of H₂, CO and CH₄ at 800 °C. After comparisons with solid oxide electrolysis cell (SOEC), the energy demands, performance and efficiency of CO-assisted SOFEC and CH₄-assisted SOFEC are investigated numerically. One important finding is that over 80% of electricity can be saved by SOFEC at a current density of 3000 A m⁻². SOFEC assisted by CO or CH₄ for steam electrolysis has better performance than SOEC, especially by CH₄. The efficiencies of 12% CO-SOFEC and 12% CH₄-SOFEC are at least, respectively, 7% and 30% higher than that of SOEC at 800 °C with the current density of below 2500 A m⁻². Finally, the effects of type of assisting-fuel, fuel composition and applied voltage are studied. It is found that CO-SOFEC shows higher anode polarization and thus lower performance than CH₄-SOFEC with the same molar fraction of fuel. It is also found that the performance of SOFEC increases with increasing proportion of assisted fuel in anode at high current density.     

Shape design and numerical analysis on a 1 MW tidal current turbine for the south-western coast of Korea

The study concentrates on the shape design and numerical analysis of a 1 MW horizontal axis tidal current turbine (HATCT), which can be applied near the southwest regions of Korea. On the basis of actual tidal current conditions of south-western region of Korea, configuration design of 1 MW class turbine rotor blade is carried out by blade element momentum theory (BEMT). The hydrodynamic performance including the lift and drag forces, is conducted with the variation of the angle of attack using an open source code of X-Foil. The optimized blade geometry is used for Computational Fluid Dynamics (CFD) analysis with hexahedral numerical grids. This study focuses on developing a new hydrofoil and designing a blade with relatively shorter chord length in contrast to a typical TCT blade. Therefore, after a thorough study of two common hydrofoils, (S814 and DU-91-W2-250, which show good performance for rough conditions), a new hydrofoil, MNU26, is developed. The new hydrofoil has a 26% thickness that can be applied throughout the blade length, giving good structural strength. Power coefficient, pressure and velocity distributions are investigated according to Tip Speed Ratio by CFD analysis. As cavitation analysis is also an important part of the study, it is investigated for all the three hydrofoils. Due to the shorter chord length of the new turbine blade in contrast to a typical TCT blade design, a Fluid Structure Interaction (FSI) analysis is also done. Concrete conclusions have been made after comparing the three hydrofoils, considering their performance, efficiency, occurrence of cavitation and structural feasibility.     

The effects of Nafion ionomer content in PEMFC MEAs prepared by a catalyst-coated membrane (CCM) spraying method

We analyzed the effects of ionomer content on the proton exchange membrane fuel cell (PEMFC) performance of membrane electrode assemblies (MEAs) fabricated by a catalyst-coated membrane (CCM) spraying method in partially humidified atmospheric air and hydrogen. When high loading Pt/C catalysts (45.5 wt.%) were used, we observed that catalytic activity was not directly proportional to electrochemical active surface area (EAS). This suggests that ionic conductivity through ionomers in catalyst layers is also an important factor affecting MEA performance. In addition, the effects of mass transport were experimentally evaluated by manipulating the air stoichiometry ratio at the cathodes. MEA performance was more sensitive to flow rates under conditions of higher ionomer content. Due to the combined effect of EAS, ionic conductivity, and mass transfer characteristics (all of which varied according to the ionomer content), an MEA with 30 wt.% ionomer content at the cathode (25 wt.% at the anode) was shown to yield the best performance.     

Impact of culture source and linoleic acid (C18:2) on biohydrogen production from glucose under mesophilic conditions

Genomic and statistical methods were used to demonstrate the effects of linoleic acid (LA) on hydrogen (H₂) production in mixed anaerobic cultures from two sources (designated as A and B). The microbial composition of the control cultures CA and CB were statistically different. Bacteroidaceae (26%) and Clostridiaceae (10%) dominated CA whereas Clostridiaceae (33%) and Bacteroidaceae (10%) dominated CB. Homoacetogens directed 42% of the electron equivalents to acetate production and decreased the H₂ yield by 50% in CA compared to CB. The maximum H₂ yields (3.11 ± 0.02 and 3.11 ± 0.07 mol H₂ mol⁻¹ glucose in LA-treated cultures ALA and BLA, respectively) were statistically the same. Cultures ALA and BLA followed the acetate-butyrate pathway while CA and CB followed propionate and homoacetogenic pathways. LA-treated and control cultures were statistically different based on the type and quantity of metabolites; the differences were also confirmed by principal component analysis (PCA).     

The first and second law analysis on an organic Rankine cycle with ejector

In consideration of the low efficiency of the organic Rankine cycle (ORC) with low-grade heat source (LGHS), an organic Rankine cycle with ejector (EORC) and a double organic Rankine cycle (DORC) based on the ORC is introduced in this paper. The thermodynamic first law and second law analysis and comparison on the ORC, EORC and DORC cycles are conducted on the cycle’s power output, thermal efficiency, exergy loss and exergy efficiency. Water is chosen as the LGHS fluid, and the same temperature and mass flow rate of the water is the standard condition for the comparative analysis on the cycles. The emphasis is on the thermodynamic performance at the maximum net power output of the cycles. The results show the power output is higher in the EORC and DORC compared to the ORC. And the cycle’s exergy efficiency could be ranked from high to low: DORC > EORC > ORC.     

Promising nature-based nitrogen-doped porous carbon nanomaterial derived from borassus flabellifer male inflorescence as superior metal-free electrocatalyst for oxygen reduction reaction

In this present study, novel hierarchical nitrogen-doped porous carbon for use as a metal-free oxygen reduction reaction (ORR) electrocatalyst is derived from borassus flabellifer male inflorescences by calcining at 1000 °C in an inert atmosphere using metal hydroxides as activating agent and melamine as nitrogen doping agent. The BET surface areas of the lithium-ion (Li-ion), potassium-ion (K-ion) and calcium-ion (Ca-ion) activated carbon are observed to be 824.02, 810.88 and 602.88 m² g^-1 respectively. Another interesting fact is that the total surface energy calculated by wicking method (73.2 mJ/m²), is found to be higher for Li-ion activated carbons. Among the prepared nitrogen-doped porous carbon, Li-ion activated system, showed an outstanding performance in ORR reaction in alkaline medium, thanks to its high surface area and notable surface activity. An incontrovertible of note that ORR half-wave potential of Li-ion activated nitrogen-doped carbon (0.90 V) is relatively higher in comparison to the commercial 20 wt % Pt/C catalyst (0.86 V). Inspite of overwhelming performance, the ORR reaction followed the preferred 4- electron transfer mechanism involving in the direct reduction pathway in all activated carbons. The ORR performance is also noticeably better and comparable to the best results in the literature based on biomass derived carbon catalysts.     

State of the art ceria-carbonate composites (3C) electrolyte for advanced low temperature ceramic fuel cells (LTCFCs)

Solid oxide fuel cells (SOFCs) are considered as one of the most promising power-generation technologies. However, the current high operation temperature (800–1000 °C) of SOFCs impedes their commercialization significantly. A key requirement for reducing the operation temperature of SOFCs is to improve the performance of the electrolyte at such low temperature. Recently, ceria-based composite materials, especially ceria-carbonate composites (3C), have been developed as competitive electrolyte candidates for SOFCs operated below 600 °C, which resulted in an emerging R & D upsurge followed up by worldwide activities. This report gives a short review on current worldwide activities on 3C for advanced low temperature ceramic fuel cells (LTCFCs), which mainly based on recent more than 70 publications since 2010. It gives an overview of materials composition and microstructure, multi-ion conduction effects, durability of the 3C materials in the areas of LTCFC or joint SOFC/MCFC filed, as well as some other novel applications of the 3C materials.     

PdCu bimetallic nanoparticles decorated on ordered mesoporous silica (SBA-15) /MWCNTs as superior electrocatalyst for hydrogen evolution reaction

In the present study, a novel electrocatalyst with excellent catalytic performance based on PdCu bimetallic nanoparticles (NPs) supported on ordered mesoporous silica and multi-walled carbon nanotubes (PdCu NPs/SBA-15-MWCNT) was prepared for electrochemical hydrogen evolution reaction (HER). For this purpose, low-cost mesoporous SBA-15 was synthesized using silica extracted from Stem Sweep Ash (SSA) as an economically attractive silica source. Mesoporous SBA-15 with unparalleled porous structure is a stable support for PdCu bimetallic NPs which prevents the accumulation of PdCu bimetallic NPs and improves its efficiency in the catalytic process. The main advantage of this strategy is low loading of bimetallic catalyst with high catalytic activity. The presence of both mesoporous SBA-15 and MWCNTs materials in PdCu/SBA15-MWCNTs/carbon paste electrode (CPE) increases the metallic active sites and the electrical conductivity of electrode which provides great performance for HER. PdCu/SBA15-MWCNTs-CPE provided small Tafel slope (45 mV dec^?1), low onset potential (~-150 mV), high current density (?165.24 mA cm^?2at -360 mV) and exchange current density (2.51 mA cm^?2) with great durability for HER in H₂SO₄ solution. Analysis of kinetic data suggests that the electrocatalyst controls HER by the Volmer-Heyrovsky mechanism. In addition, studies showed that the presence of sodium dodecyl sulfate (SDS) in electrolyte can decrease the potential of HER and increase the current density.     

Sulphur-reduced graphene oxide composite with improved electrochemical performance for supercapacitor applications

In this research, carbon nanorods/fibers materials were successfully synthesized from sulphur-reduced graphene oxide (RGO-S) composite by using an improved Hummers' method. Morphological, structural, compositional and textural characterization of the composite material were obtained via scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), respectively. The electrochemical performance of the composite sample as a promising supercapacitor electrode revealed a peak specific capacity of 113.8 mAh g⁻¹ at 0.5 A g⁻¹ estimated via GCD curves in 6 M KOH aqueous electrolyte. The half-cell could retain a columbic efficiency of about 98.7% with a corresponding energy efficiency of about 98.5% over 2000 constant charge/discharge cycle at a specific current of 5 A g⁻¹. Remarkably, an assembled hybrid device with carbonized iron cations (C-FP) and the RGO-S composite delivered high energy and power densities of 35.2 Wh kg⁻¹ and 375 W kg⁻¹ at 0.5 A g⁻¹ within a 1.5 V operating potential, respectively. A good cycling stability performance with an energy efficiency of 99% was observed for the device for up to 10,000 cycling at a specific current of 3 A g⁻¹.     

Ni activated Mo2C nanoparticles supported on stereotaxically-constructed graphene for efficient overall water splitting

Exploring cost-effective, high-efficiency and stable electrocatalysts for overall water splitting is greatly desirable and challenging for sustainable energy. Herein, a novel designed Ni activated molybdenum carbide nanoparticle loaded on stereotaxically-constructed graphene (SCG) using two steps facile strategy (hydrothermal and carbonization) as a bifunctional electrocatalyst for overall water splitting. The optimized Ni/Mo₂C(1:20)-SCG composites exhibit excellent performance with a low overpotential of 150 mV and 330 mV for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively to obtain a current density of 10 mA cm^?2 in 1.0 M KOH solution. In addition, when the optimized Ni/Mo₂C(1:20)-SCG composite is used as a bifunctional electrode for overall water splitting, the electrochemical cell required a low cell voltage of 1.68 V at a current density of 10 mA cm^?2 and long-term stability of 24 h. More significantly, the synergetic effects between Ni-activated Mo₂C nanoparticles and SCG are regarded as a significant contributor to accelerate charge transfer and promote electrocatalytic performance in hybrid electrocatalysts. Our works introduce a novel approach to design advanced bifunctional electrodes for overall water splitting.